Pedigree analysis of selected lines of Merino sheep. 1. Inbreeding

Abstract

Four pairs of selection lines of Merino sheep consisting of 100 ewes and 5 rams were established at the Trangie Agricultural Research Centre in 1951. Each line was selected (mass selection) for the appropriate character and direction (Plus or Minus) for a period of 18 years (5 generations). The characters subjected to selection were clean fleece weight, crimp frequency, weaning weight, and skin folds. The average inbreeding coefficients calculated from the pedigree data of the those selected lines ranged from 3.79% in the Fleece Plus line to 9.09% in the Folds Plus line, corresponding to average annual inbreeding rates of 0.211% and 0.505%, respectively. Generally, there were similar fluctuating patterns in the means of inbreeding coefficients across years, with higher variation in the early years, and different rates of inbreeding in different flocks. The only exception was the Fleece Plus flock with a trend to increased variation across years, higher fluctuations in average inbreeding coefficient in final years, and inbreeding, which was not located in the 95% confidence interval of the inbreeding in flocks simulated for an infinitesimal model. Although the Folds Plus line showed inbreeding within the 95% confidence interval of simulation inbreeding, it was located at the higher end of the confidence interval, due to natural selection. Excellent agreement was found between the simulated flocks inbreeding (6.54%) and predicted inbreeding from an adaptation of the formula of JA Woolliams, NR Wray, and R Thompson (1993) (6.44%).